Reductive decomposition of galena (PbS) using Cr(II) ionic species in an aqueous chloride medium for silver (Ag) recovery

The aqueous reductive decomposition of a galena flotation concentrate to lead and hydrogen sulphide in order to expose silver has been investigated using an acidified chloride solution of Cr(II) ionic species. The results obtained show a strong dependence on pH of the reductive leaching process. The leaching of galena at pH values lower than 0.43 proceeded via both chemical and electrochemical paths. At these pH values (<0.43), the electrochemical contribution to the reductive decomposition reaction was less significant. Increasing the temperature above 90°C enhanced the reaction kinetics of the leaching process considerably. Diffusion control was operative in the temperature range 70°C–105°C where an activation energy value of 18.8 kJ/mol was determined. Subsequent to the reductive decomposition of the sulphidic matrix, silver recoveries of up to 95% in cyanide solutions were achieved. Recovery (85%) of lead was obtained by first digesting the lead containing residue followed by chilling of the pregnant liquor to temperatures below 10°C so as to crystallize lead nitrate.